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LLC Resonant SMPS problems


Aug 3, 2009
Aug 3, 2009

I think i have an SMPS that cannot be done with an LLC Resonant converter.
It is as follows:

Vin = 385VDC
Vout = 80V
Iout = 4A

I tried to design an LLC Resonant converter for the above spec.......i wished to use an external "leakage" inductance.
-Therefore i used the design procedure of pages 1 to 3 of the following.....

However, no matter what values are chosen for frequency, Cr, Lr, Lp, ........I cannot get a workable design.

The problem is that the resonant frequency "f0" occurs at the peak gain frequency.

...this is a massive problem because:-
1....You need to operate at the resonant frequency "f0"
2....You need to operate significantly below the peak gain frequency (otherwise there is a danger of going into the capacitive region where hard switching occurs) now you can see the problem........i need to operate at the resonant frequency, but cannot because its the peak gain frequency.

Have i picked a load and input voltage that can't be served by a LLC Resonant frequency?